Although solar radiation initially is unpolarized when entering the
Earth's atmosphere, it is polarized by scattering processes with
molecules, water droplets, ice crystals, and aerosols.
Hence, measurements of the
polarization state of radiation can be used to improve remote sensing
of aerosols and clouds.
The analysis of polarized radiance measurements requires
an accurate radiative transfer model.
To this end, a new efficient and flexible three-dimensional Monte
Carlo code to compute polarized
radiances has been developed and implemented into MYSTIC
(Monte Carlo code for the
phYSically correct Tracing of photons In Cloudy atmospheres).
The code has been extensively validated against published benchmark results.
The polarized downwelling radiation field is calculated
for various aerosol types showing the high sensitivity of
polarized ultraviolet radiances to the particle microphysics.
Model simulations are compared to ground based measurements and found
to be qualitatively in good agreement. Quantitative differences can be
attributed to
the assumed aerosol models based on the OPAC aerosol database, which does not
include exactly the types of aerosols that have been observed.
This comparison to the measurements shows that there is a high
potential to retrieve
information about the aerosol type from polarized radiance measurements.